The invention relates to a radio-interrogated passive sensor.
Surface-wave arrangements relevant to the invention in connection with sensors are known from International Reference WO 93/13495, International Reference WO/CH93/00252, U.S. Pat. No. 3,273,146, U.S. Pat. No. 4,725,841, U.S. Pat. No. 4,620,191.
The cited publications describe methods of building and operating various surface-wave arrangements, which all have the common factor that, by means of an interdigital transducer, surface waves are produced in a piezoelectric substrate body from an electrical signal, and these surface waves as a rule propagate essentially perpendicularly to the interdigital alignment of the transducer electrodes at the surface of the substrate body. With a second interdigital transducer, which may even be the transducer already described above with double mode operation, it is possible to recover a characteristically altered radiofrequency signal from the surface wave. As described in the prior art, such a surface-wave arrangement may comprise yet further structures such as reflector structures, further transducer structures, etc. which, for example, may also have dispersive arrangement of the reflector/transducer fingers, encoded arrangement of the fingers, and the like.
An important aspect and subject of the present invention is a radio-interrogated surface-wave technology sensor which is passive, that is to say requires no (DC) power supply, and to which a radiofrequency signal, for example a burst pulse, an FM-CW signal, a chirped pulse or the like, is transmitted from a remote radiofrequency transmitter.
The surface-wave arrangement of the sensor part, that is to say its input transducer structure is for this purpose equipped with an antenna for radio reception of this transmitted pulse. A corresponding antenna, which is connected to a second transducer structure of the surface-wave arrangement, or, in the case already mentioned of a transducer structure with double-mode operation, is the same antenna, is used to transmit back the impulse-response signal of the surface-wave arrangement, which is to be received in a remote receiver. The impulse-response signal transmitted back is as a rule different than the signal received by the surface-wave arrangement, namely according to the current-strength measurement to be determined, and this is actually because of corresponding physical action on the surface-wave arrangement.
Radio-interrogated surface-wave arrangements are already used, for example, in toll systems on roads, in inroad tunnels or the like, but in this case the detection of preprogrammed individual encoding of the impulse-response signal for object identification is involved. Radio-interrogated surface-wave arrangements have also been used in metrology, these arrangements being as a rule constructed as delay lines, and measures being taken for the purpose of measurement such that the measured quantity to be determined in the surface-wave arrangement causes a change in the propagation time in the acoustic wave. This change in propagation time may be based on an electric field (oriented transversely to the propagation direction of the surface wave) in the substrate body, this field producing, for example by piezoelectric effect, a change in propagation time in the corresponding partial region of the substrate body (European Reference 0166065). By way of example, a temperature sensor using the change in the wave propagation time is known (European Reference 0465029). An arrangement which exploits an impedance change of an organic layer applied to the surface of the substrate body of the surface-wave arrangement is suitable for measuring surface loading of this layer, for example with a chemical substance to be identified/quantitatively measured (Electronics Letters, Vol. 23 (1987) No. 9 pp. 446/447). A relevant pressure meter is also known, in which the mechanical property of the body, for example flexion, altered as a function of the pressure in the material of the substrate body of the surface-wave arrangement, causes a change in the propagation time of the acoustic wave and renders it usable for determining the measured value (Proceedings IEEE, Vol. 64, (1976) pp. 754-6). However, in the case of the arrangements last mentioned here, a remote interrogation by radio is not provided.